A combination of a piezo-electric position actuator and a piezo-electric force sensor, which actuator and sensor are positioned in series, is known from the technical literature, and is often referred to using the term. “Smart Disc”. In every Smart Disc, there is a control system that receives an output signal from the force sensor—in the form of a voltage signal—as an input signal for the control system and, in reaction hereto, generates a control signal for the attendant actuator. The operation of the actuator can thus be aimed at opposing the force observed by the sensor, e.g. as caused by accelerative forces associated with small vibrations, which phenomenon can be usefully exploited in opposing small vibrations. The relationship existing between the control signal generated by the control unit and the input signal received by the control unit is also referred to using the term “controller transfer”, with a certain frequency-dependent characteristic and a certain amplification factor (also referred to using the term “gain”).
A particle-optical device according to the opening paragraph is known from European patent application EP 1225482 A1. Said document describes a lithographic device that uses a beam of UV, electrons or ions to process a wafer for integrated semiconductor circuits. To this end, use is made of an optical system with a lens for generating and focusing a beam of particles onto a desired position on a wafer. The optics—or more specifically their lens—are supported on a horizontal main plate via three lens supports. Each of the lens supports comprises a pair of Smart Discs. The main plate can be regarded as being connected to the fixed world via air springs and dampers with a typical eigenfrequency of the order of 1 Hz. Underneath the lens is located a wafer that is supported by a wafer table, which can manipulate the wafer in the horizontal plane and also in the vertical direction, with the purpose of following vibrations in the main plate caused by the resilience of said air springs and dampers. To this end, one or more interferometers are provided, comprising part of a control circuit that ensures that the vertical distance between the main plate and the wafer remains constant, so as to achieve a correct focus.
The lens typically has a first eigenfrequency located in the range 50–150 Hz. Resonance of the lens can therefore arise as a result of environmental acoustic noise or floor vibrations, e.g. generated by apparatus surrounding the device. Such vibrations can ultimately lead to a situation whereby the accuracy of the (horizontal) positioning of the beam of particles and/or the focusing of the beam of particles on the wafer is no longer adequate. By employing the Smart Discs (of which there is a total of six, corresponding to the six degrees of freedom of the lens), the vibrations resulting from this resonance are actively damped, as a result of which an improved accuracy of the focusing of the beam of particles on the wafer can be realized.